Bliniking shadow on rendering result
Well since nothing works im posting something that may work. sellect the object you have this problem apply a smooth shader in it and then add a subdivision modifier change it to simple from catmull-clark.Then change subdivisions to something big like 4-5.
And check again to see if it works.If this still doesnt work add one more subdivision modifier but not simple this time.And check again.
This wont work for sure*